Support single-source changes with `call_fixture`

# ./changes.yml
---
- type: user
  params:
    id: 1

- const: DEFAULT_USER_ID
  value: 1

- url: https://example.com/users/default
  method: get
  responses:
    - body:
        id: 1
        name: Andrew

before { call_fixture "#{__dir__}/changes.yml" }